1. Field of the Invention
The present invention relates to an electroluminescent element comprising an anode, a cathode, and a layer including organic compounds (hereinafter, an electroluminescent layer) which emit light by applying current through the pair of electrodes; and a light-emitting device which comprises the electroluminescent layer. More specifically, this invention relates to an electroluminescent element which exhibits white light emission, and a full color light-emitting device comprising the electroluminescent element.
2. Related Art
An electroluminescent element comprises an electroluminescent layer interposed between a pair of electrodes (anode and cathode). The emission mechanism is as follows. Upon applying voltage through the pair of electrodes, holes injected from an anode and electrons injected from a cathode are recombined with each other within the electroluminescent layer to result in the formation of molecular excitons, and the molecular excitons return to the ground state while radiating energy to emit photon. There are two excited states possible from organic compounds, a singlet state and a triplet state. It is considered that light emission is possible through both the singlet state and the triplet state.
Although an electroluminescent layer may have a single layer structure comprising only a light-emitting layer formed by a light-emitting material, the electroluminescent layer is formed to have not only a single layer structure comprising only a light-emitting layer but also a lamination layer structure having a bole injecting layer, a hole transporting layer, a hole blocking layer, an electron transporting layer, an electron injecting layer, and the like which are formed by a plurality of functional materials.
It is known that color tone can be appropriately changed by doping tiny amounts of fluorescent substances (typically, at most approximately 10−3 mol % based on the value of host substances) into host substances within the light-emitting layer. (For example, refer to Japanese Patent Publication No. 2,814,435.)
Besides, the following are known as methods for changing color tone. Blue light emission obtained from a light-emitting layer is used as a light emission source, and the obtained emission color is converted into desired color within a color changing layer formed by color changing materials (hereinafter, CCM method). Alternatively, white light emission obtained from a light-emitting layer is used as a light emission source, and the obtained emission color is converted into desired color by a color filter (hereinafter, CF method).
However, in case of adopting the CCM method, there has been a problem in red color since color conversion efficiency of from blue to red is poor in principle. In addition, there has been a problem that the contrast becomes deteriorated from light emission in pixels due to outside light such as sunlight since color conversion materials are fluorescent materials. Therefore, it is considered that CF method without such problems is preferably used.
In the case of using CF method, an electroluminescent element exhibiting white light emission (hereinafter, white light emission element) having high luminance is required since much light is absorbed in a color filter.
With respect to the white light emission element, elements formed by various materials to have various configurations have been reported. It is quite important to control the balance of emission color (white balance) since white light emission is obtained by a plurality of materials, each of which emits different color, meanwhile it is difficult to do that.
For example, in the case that white light emission is obtained by mixing a plurality of materials, each of which exhibits emission in blue, green and red colors, it has been reported that an emission peak intensity of each the materials is different depending on current density. (For example, refer to Brien W. D'Andrede, Jason Brooks, Vadim Adamovich, Mark E. Thompson, and Stephen R. Forrest, Advanced Material (2002), 14, No. 15, Aug. 5, 1032-1036 (FIG. 2)) In case of forming such element, peak intensity of each emission color changes in different rates.
Consequently, it becomes extremely difficult to control white balance which is adjusted by the parameter which determines the peak intensity of these emission colors.